Shock absorbent strut for aircraft



l 'I rem. 7, 1939. J, H. oNxoNs SHOCK ABSGRBENT STRUT FOR AIRCRAFT Filed Aug. l5, 1936 Patented' Feb. 7, 1,939

UNITED STATES PATENT OFFICE Appueaaon August 15, 1936, serial No. 96,270 In Great Britain August 15, 1935 s claim.

This invention relates toresilient struts, particularly' 'for aircraft, of the kind in which a' plunger tube is vslidably mounted within a cylinder tube, each being closed at a position remote 5 from its inner end to provide an internal space of variable volume containing a liquid, such as oil, and-gas under pressure, such as air, the liquid during relative reciprocation of the tubes'in one or both directions being caused to pass through 10 one or more restricted openings for damping this movement of the tubes, while vthe compressed gas provides the whole of the resilience normally required,.the strut of course, being capable of carrying a substained normal load without bel5 coming completely closed.

It is the object of the ,present invention to provide an improved construction of strut which is capable of proper operation irrespective of its position with regard to the vertical, said strut 20 accordingly being satisfactory for use with various designs of undercarriage or like structures, and even in cases where complete or partial inversion has to be allowed for as in some designs of retractable undercarrlage and in machines l5 adapted for inverted flying. r

.The invention further seeks to provide an improved form of strut construction which is extremely rigid by comparison with its weight and aerodynamic drag, the general design of said i strut being such that the rate of compression and other characteristics can readily be modified during the assembly ofthe parts without changing the dimensions of the latter or of the strut unit as a whole.

l The invention accordingly provides a strut of the kind referred to in which there is provided a oating piston which is free in relation to the plunger and cylinder tubesyand which is arranged to constitute the boundary between the L0 liquid and the compressed gas. If desired, a lfixed piston which is anchored relatively to the cylinder tube may also be provided within the plunger tube for the purpose of enabling a positive damping effect to be obtained during the 5 expansion or rebound movement o! the strut.

Moreover, in a resilient strut oi.' the-kind reierred to comprising a unit, the whole length of which is constituted by two continuous lengths of tube serving as plunger and cylinder respec- 0 tively, according to a further feature of the invention the internal space containing the compressed gas is bounded by a plug member formed separately from the corresponding tube and of greater axial thickness than the thickness of the s wall of said tube, which plug member is disposed intermediate the ends of the tube to which it is attached and which it serves to close in a :duid tight manner. 'Ihus the internal space containing the liquid and compressed gas may be bounded at its two ends by plug members, each of which 5 is formed separately from the tubes and is disposed intermediate the ends of that tube to which it is attached, the arrangement being such that the two tubes together constitute the whole length of the strut as governed by the general design and size of the undercarriage or equivalent, while a. portion only of the total length can readily be utilized for the liquid andV compressed gas space. This enables the improved struts to be constructed from comparatively light tubing, and as the whole cross-sectional area of the space within the tubes is` utilized in an eiicient manner, the diameter, and more especially the weightfor a given loading, can be made very much less than is possible with struts known hitherto. 'I'he 20 internal space containing the liquid and compressed gas may conveniently be bounded at one end at least by a fixed plug member tted within one of the tubes intermediate its ends and provided with a peripheral packing composed of soft material, such as rubber, said packingii desired being ln the form of an annular cup washer packing of L-shape in radial section.

v For the purpose oi supporting the freely mounted piston serving as the boundary between the liquid and the gas under pressure, an axial stem may be provided by which said piston is maintained inalignment with the axis of the ,plunger tube.

'I'he invention is illustrated in the accompany- 35 ing diagrammatic drawing in which:-

Figure 1 is a longitudinal sectional elevation through one form of strut; and

Figure 2 is a similar view showing a modied construction. 40

It will be understood that, in order to make the general operation clear, the diameters of the various parts lshown in the drawing have been somewhat exaggerated and that as designed for present day aircraft the thicknessof the tubes andtheir diameters'can generally be advantageously reduced by comparison with the stroke and overlap of the plunger tube within the cylinder tube.

Referring rstly to Figure 1, the cylinder tube l siidably mounted within thev bush l2 is a plunger tube I4 carrying at its inner end a substantially cylindrical body I5 serving as a .plunger head. The latter is provided upon its periphery with any suitable form of leak-preventing packing indicated at I6, and is hollowed out at its centrepart for the accommodation of an annular plate I1 serving as a so-called flutter valvev for producing controlled liquid' damping, said plate I1 conveniently being maintained in position by the inwardly overhanging edge of a retaining ring I8. The lower wall of the plunger head I5 is provided with a circular series of comparatively large apertures I9, which are adapted to be covered by the 'plate I1 and with a small central opening 20 disposed in lregister with a larger central opening 2| in the plate I1. It will thus be seen that when the fluid pressure below the piston head is greater than that above, as seen in Figure 1, the uid will flow through the holes I9 and lifting the plate I1 will passr relatively freely through the central aperture 2|. On the other hand when the pressure within the plunger tube I4 is the greater the plate I1 will be held iirmly downwards, rand-the fluid can then only pass through the restricted orifice 20, since the holes I9 are closed. f

At a position below the plunger head I5 (as seen in Figure l) the cylinder tube I0 is closed by a plug member 22 whichmay conveniently be disposed intermediate the two ends of the tube I0 so that the strength' and rigidity of this portion of the strut is maintained. This plug member 22 is held in position by means of say three diametral pins, rivets, bolts or equivalent 23, while a fluid tight joint is obtained by the use of a packing ring indicated at 24. The plug member 22 is provided with a passage 25 through which oil .is forcedy into the interior of the cylinder, a screwthreaded closure plug 26 conveniently being provided. An annular buffer 21, composed for example of rubber, rests upon the plug member 22 and serves as a resilient stop for arresting inward movement of the plunger tube I4l, while a similar ring 28 surrounding the plunger tube I4 and supported by a shoulder 29 upon a spacing tube 3U engages with the inner part 3l of the bush I2 when the limit of outward movement of the plunger tube I4 is reached.

Adjacent the outer end of the plunger tube I4 anther plug member is provided, and is indicated at 32, said plug member being provided with a packing ring 33 and being restrained against l'outward movement by a bell-shaped fitment 34 which is provided lwith a lug 35 serving for the external attachment of theplunger tube I4 to the aircraft fuselage or edivalent, transverse pins or bolts 36 being provided for holding the fitment 34 in position. The plug member 32 is moreover tted with an air inlet valve 31 through which compressed air is passed so as to build up ing as the boundary between compressed air or other gas in the space 40 and oil or other liquid in thewspace 4I within the plunger tube I4 and space l42 within the cylinder tube I0.

When in flight, the strut is fully extended.

If a leak of oil should-occur, the air pressure would l force the floating piston against the plunger head thus relieving the liquid in thecylinder fromvall further pressure,`and allowing there t0 be maintained within the cylinder suicient liquid to ensure a safe landing.

It will, of course, be appreciated that the total space 40, 4I and 42 varies in volume according to the relative positions of the tubes I0 and I4 and that this space is bounded by the two plug members 22 and 32. A` sleeve 43 is fitted Within the upper part of the space 40 and is utilized as a stop for the floating piston 38, said sleeve conveniently being of somewhat soft material, such as rubber. It will be appreciated also that at the' lower end (not shown) of the cylinder tube I0 a convenient tment is provided whereby said tubecan be attached to the axle or other equivalent 'part of an undercarriage, so that in the example shown in Figure 1 the main part of the length of the strut is constituted by the cylinder tube I0, l

The .procedure of preparing the strut for use after-installation is very simple, and consists in opening the air valve 31 then fully compressing the strut until the plunger head I5 rests upon the buffer 21 and pumping in oil through the passage 25 until the piston 38 reaches the lower end of the sleeve 43. The plug 26 is then replaced and air is pumped in through the valve fitting 31 until the requisite pressure in the space 4U is attained. This air pressure serves to provide the whole of the resilience of the strut whereas the desired damping effect for reducing oscillation is obtained in the known manner by the action of the utter Valve I1.

A somewhat similar general, arrangement but of diierent construction is shown in Figure 2 in which the relative disposition of the parts is reversed, the cylinder tube I0 containing the oil in this case constituting what is normally the upper part of the strut. Mounted within the inner or lower endl I of the tube I8 is the plunger tube I4 carrying at its upper end a plunger head I5 arranged as in the previous example, except Athat it is inverted.

The upper plug member, indicated at 45, in this case supports acoaxial rod or tube 46carrying at its outer end a circular plate 41 serving as a xed piston i. e. a piston which is fixed relative to the cylinder tube IIJ, said piston 41 being provided with a number of small apertures 48 and, if desired, a comparatively large working clearance between its periphery and the interior of the tube I4 so as to allow fluid to pass from one side to the other of said fixed piston 41. An aperture 49 formed in the centre of the plunger head I5 also has a prescribed worln'ng clearance with regard to the exterior of the rod or tube 46, and this serves to provide the constricted pas- 'sage through which the liquid has to ow for producing the necessary damping action. The plunger head I5 is also provided with comparatively large apertures I9 as before, while the fiutter plate I1,which is formed with a large centralihole 21",'. is slidably mounted within a cavity 50, but normally closes the apertures I9 under the action of a coiled compression spring 5I. A resilient stop Iring 21 is provided as previously within the cylinder tube I0 and is engaged by the plunger head I5 when the strut is compressed to the maximum extent, but the rebound movement plunger tube I4 and arranged to engage with the fixed piston 41. A floating or free piston 38 is again provided within the plunger tube I 4, and

is arranged to separate thel oil within the space 4| from the compressed air in the space 48, said free lpins 6|. tended to bear against the usual end fitting (not piston, if desired, being provided with an axial stem which is indicated at 53, and is slidably mounted within a bore in the member 46, thus maintaining the free piston in a truly radial plane without the necessity of unduly increasing its length and weight.

The arrangement shown in Figure 2 having the fixed piston 41 is mainly advantageous on account of the fact that, during both expanding and contracting movement of the strut, a positive liquid pressure is present for forcing the liquid past the flutter valve, the extent of this positive pressure depending upon the force exerted axially upon the strut, and in no circumstances is it.

possible for a vacuum or suction eifect'to bev produced under the action of an excessively` violent extension movement of the two tubes Il) and i4. It will be appreciated moreover that the fixed piston 41 has a steadying effect upon the movement of the free piston 38, as the volume of liquid between these two pistons only varies to a slight extenton account of the difference between the cross-sectional area of the cylinder tube Ill and the cross-section area of the plunger' chorage screws or equivalent (not shown) are' of course provided for preventing endwise displacement of the plug member 45 and an oilfilling, plug 26 closing an inlet passage 25 is fitted as in the previous example.

The lower plug member, indicated at 59, comprises a substantially cylindrical block of metal which fits Within the plunger tube I4, and is prevented from endwise displacement by means of a tube 60 which is secured by one or more diametral Alternatively the tube 60 may be exshown) of the s trut to resist the end thrust on the plug 59. The plug member 59 is fitted with a Valve 62 through which air under pressure can be passed into the space 40, an eflicient seal between said member 59 and the interior of the plunger tube I4 being obtained by a cup washer packing 63 which is conveniently formed from rubber, and is of L-shape in radial section. The two lips 64 and 65 are forced by the air pressure into firm engagement with the interior of the tube I4 and the upper -surface of the plug member 59 respectively, and a reliable seal is thus obtained.

The invention thus provides an extremely simple construction of resilient strut which isv particularly suited to the needs of aircraft, as it is 'very light in Weight and small in diameter by comparison with its permissible loading. This to a large extent is due to the fact that ordinary drawn tubingof steel or light alloy can readily be used in its manufacture, and that the great strength of this tubing is utilized to the best advantage to resist bending moments as the only essential joint which is positioned intermediate the ends of thestrut is the sliding joint between the cylinder tube and the plunger tube. This joint, of course, can easily be made sufliciently rigid by arranging that the tubes shall overlap to an adequate amountwhen the strut is fully extended. Further, the use of the free piston enables the compressed gas to be confined to the plunger tube. thus reducing leakage to a minimum, and moreover it permits the sliding packing of the plunger head to be permanently immersed in oil or other working liquid irrespective of the attitude in which the strut is placed thus avoiding sliding air seals. This not only tends further to reduce leakage but also maintains the sliding surfaces of the plunger tube and cylinder tube in a well lubricated condition. Finally, the improved strut can be very satisfactorily employed in retractable undercarriages of all forms, and is of particular utility in those cases where the strut becomes inverted during the action of folding the undercarriage.

What I claim isz- 1. A resilient oleo-pneumatic strut comprising a cylinder tube, a plunger tube, said tubes being closed at a part remote from their inner ends, said tubes being slidably engaged and defining a Working space of variable volume, a piston head on the inner end of the plunger tube, a floating partition piston dividing 4the space within the plunger into compartments, one'of said compartments for the accommodation of compressed air and the other of said compartments forthe accommodation of oil, said plunger head having a construction formed therein to limit the passage of oil between the plunger and the cylinder, a fixed piston disposed within the `oil-filled space between the floating partition piston and the piston head and providing a damping effect, and a stem secured to the cylinder and extending through the piston head for supporting the fixed piston.

2. A resilient oleo-pneumatic strut as claimed in claim 1 having at the back of the piston head an annular buffer which is engaged by the fixed piston as the strut extends, thereby resiliently limiting the extension movement of said strut.

3. Aresilient oleo-pneumatic strut comprising va cylinder tube and a plunger tube each closed at a part remote from its inner end, said tubes being slidably engaged to define a working space of variable volume, a piston head on the inner end of the plunger tube, a floating partition piston dividing the space within the plunger into two compartments, one of said compartments being for the accommodation of compressed air and the other of said compartments being for the reception of oil, said plunger head having a constriction movement to limit the passage of oil between the plunger and the cylinder rduring movement of the tubes, a fixed piston comprising a circular yplate perforated with aiplurality of constricted openings, said fixed piston being disposed within the oil filled space between the oating partition piston and the piston head and providing a damping effect during relative movement between the cylinder tube and plunger tube,

and a stem secured to the cylinder and extending through the piston head for supporting the fixed piston.

4. A resilient oleo-pneumatic strut comprising a cylinder tube and a plunger tube each closed at a part remote from its inner end, said tubes being slidablyngaged and defining a working space of variable volume, a piston head on the inner end of the plunger tube, a floating piston dividing the space within the plunger into two compartments, one of said compartments for the accommodation of compressed air and the other 0f said compartments for oil, a central aperture formed in the plunger head, an axial stem carried by the cylinder tube and extending through the piston head into the plunger tube and defining a clearance between said axial stem and said piston head, said clearance constituting a constriction through which the oil is caused to pass, and a iixed piston comprising a circular plate carried by said stem and disposed within the oil lled space between the floating piston and the piston head.

. 5. A resilient oleo-pneumatic strut comprising a cylinder tube and a plunger tube each closed at a part remote from its inner end, said tubes being slidably engaged to dene a working space of variable volume, a piston head on the inner end of the plunger tube, a floating partition piston dividing the space within the plunger tube into two compartments, one of said compartments for compressed air and the other of said compartments for oil, a resilient sleeve within the plunger tube to limit the movement of the floating piston toward the Vouter end of the compressed air compartment, a xed piston disposed within the oil filled space between the iloating partition piston and the piston head and serving as a damper for movement between the tubes, and a stem secured to the cylinder vand extendingthroughvthe piston head for supporting the xed piston. Y

6. A resilient oleo-pneumatic strut comprising a cylinder tube and a plunger tube, a removable plug member in each tube at a position vremote from its inner end, said tubes being slidably en.

gaged to define a working space of variable volume, a piston head on the inner end oi.' the plunger tube, a floating partition piston dividing the space within the plunger .into two isolated compartments, one for compressed air and the other for oil, a fixed piston disposed within the oil iilled compartment between the piston head and the iioating piston, and a stem secured to the cylinder and extending through the piston head for supporting the fixed piston.

7. A resilient oleo-pneumatic strut as claimed in claim 6wherein the removable plug member in the cylinder tube comprises a pair of elements,

means whereby the. elements are clamped together axially, and a rubber ringy which is thereby compressed outwardly into Huid-tight contact with the tube.

8. A resilient oleo-pneumatic strut comprising a cylinder tube, a plunger tube slidably mounted therein, a piston head at the inner end of thev plunger tube, a removable closure plug at the other end of said tube, a removable plug closing the outer end of the cylinder tube, an axial stem carried by the cylinder closing plug and extending through an aperture in the piston head, a perforated plate serving as a iixed piston carried upon the end of the stem within the plunger tube, a floating partition piston disposed within the plunger tube between the xed piston and the closure plug for the plunger tube, and a flutter valve plate carried by the piston head and adapted to close a series of main openings through the piston head during the extension movement of the strut thereby constraining the liquid damping medium to pass throughthe constricted clearance passage between the stem. of the iixed piston and the hole in the piston head through which said stem passes.

` JOHN HENRY ONIONS` 

